Brūno (also: Bruno) Haralds Kalniņš (7 May 1898 – 26 March 1990) was a Latvian social democratic politician and historian.

He was the son of prominent social-democratic politicians Pauls Kalniņš and Klāra Kalniņa.

Kalniņš was one of the leading figures of the social-democratic youth movement in interwar Latvia, and an elected member of the Saeima.

As a key social-democratic activist, he was arrested and exiled to Sweden as a result of the authoritarian coup d'état by Kārlis Ulmanis in 1934.

After World War II he lived in Sweden, where he wrote several books on Baltic and Russian history and helped re-form the Latvian Social Democratic Workers' Party in exile.